Photoproduction of H-2 gas has been examined in sulfur/phosphorus-deprived Chalmydomonas reinhardtii cultures, placed in photobioreactors (PhBRs) with different gas phase to liquid phase ratios (V-g.p/V-l.p). The results demonstrate that an increase in the ratio stimulates H-2 photoproduction activity in both algal suspension cultures and in algae entrapped in thin alginate films. In suspension cultures, a 4x increase (from similar to 0.5 to similar to 2) in V-g.p/V-l.p in a 2x increase (from 10.8 to 23.1 mmol l(-1) or 264-565 ml l(-1)) in the total yield of H-2 gas. Remarkably, 565 ml of H-2 gas per liter of the suspension culture is the highest yield ever reported for a wild-type strain in a time period of less than 190 h. In immobilized algae, where diffusion of H-2 from the medium to the PhBR gas phase is not affected by mixing, the maximum rate and yield of H-2 photoproduction occur in PhBRs with V-g.p/V-l.p above 7 or in a PhBR with smaller headspace, if the H-2 is effectively removed from the medium by continuous flushing of the headspace with argon. These experiments in combination with studies of the direct inhibitory effect of high H-2 concentrations in the PhBR headspace on H-2 photoproduction activity in algal cultures clearly show that H-2 photoproduction in algae depends significantly on the partial pressure of H-2 (not O-2 as previously thought) in the PhBR gas phase. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.